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AS Bio - Can someone please explain the Modification of Histone Proteins?

Am I right in thinking that...

Modification of histones for example, by removing acetylgroups, makes DNA wrap tighteraround the histone proteins (chromatin becomes highly condensed). This prevents transcription to mRNAas RNA polymerase can’t bind to DNA. So the gene is NOT expressed.

^Is this correct or have I gotten all my notes down wrong?
Reply 1
Avatar for alkyone
alkyone
12
Histone acetylation is on lysine residues that are positively charged and interact with DNA. Acetylation leads to relaxation so enzymes can access the template DNA to make mRNA so you get expression of genes in that area.
(edited 7 years ago)
Original post by Philip-flop
Am I right in thinking that...

Modification of histones for example, by removing acetylgroups, makes DNA wrap tighteraround the histone proteins (chromatin becomes highly condensed). This prevents transcription to mRNAas RNA polymerase can’t bind to DNA. So the gene is NOT expressed.

^Is this correct or have I gotten all my notes down wrong?


Yeah, it is right that the DNA is wrapped tigther around by histone and yeah the chromatin becomes condensed what leads to the chromosome. So far, so good. In terms of transcription, you are wrong.
(edited 7 years ago)
Reply 3
Avatar for alkyone
alkyone
12
Original post by Kallisto
Yeah, it is right that the DNA is wrapped tigther around by histone and yeah the chromatin becomes condensed what leads to the chromosome. So far, so good. In terms of transcription, you are wrong.


But acetylation on lysines neutralises the positive charge meaning that the binding to the phosphate backbone of DNA is less efficient, meaning that the DNA is less well bound.
Original post by alkyone
But acetylation on lysines neutralises the positive charge meaning that the binding to the phosphate backbone of DNA is less efficient, meaning that the DNA is less well bound.


Oh boy, I have really forgotten to consider the acetylation, although it was even written in bold print. *sigh* Next time I should a bit take care before I give an answer.
Reply 5
Avatar for alkyone
alkyone
12
Original post by Kallisto
Oh boy, I have really forgotten to consider the acetylation, although it was even written in bold print. *sigh* Next time I should a bit take care before I give an answer.


Haha I did the same initially because for some reason I was just thinking methylation then once I hit reply I thought oops, acetylation. Which is why I had to edit.
Original post by alkyone
Haha I did the same initially because for some reason I was just thinking methylation then once I hit reply I thought oops, acetylation. Which is why I had to edit.


Good to know that you made a mistake too. By all means your edited explanation is right: thanks to the the interaction by an acetylgroup and the charge you mentioned, the gen expression is restricted.
Original post by Kallisto
Yeah, it is right that the DNA is wrapped tigther around by histone and yeah the chromatin becomes condensed what leads to the chromosome. So far, so good. In terms of transcription, you are wrong.


Ok, in that case I'm confused. Here is a quote taken from the Edexcel Endorsed textbook (Salters Nuffield AS/A level Biology)

"The attachment for example, methyl groups (-CH3) to the DNA of a gene, usually to cytosine, prevents transcription to mRNA, by stopping the RNA polymerase from binding. The modification of histones by addition, for example of, methyl or acetyl groups, affects how tightly the DNA is wrapped around the histone. When wound tightly, the genes are inactive: they cannot be transcribed to mRNA. The gene therefore cannot make protein; it is 'switched off'."

Can you tell me what part "In terms of transcription" that I'm wrong please. I'm really stuck :frown:
Original post by Philip-flop
x


Don't be afraid, everything is fine. I have given an answer for ordinary histones, so without considered acetyl groups or methyl groups and so without modification. Have not read your post through, I am so sorry. Read alkyone's post (#2), that is a good explanation.
Reply 9
Avatar for alkyone
alkyone
12
Original post by Philip-flop
Ok, in that case I'm confused. Here is a quote taken from the Edexcel Endorsed textbook (Salters Nuffield AS/A level Biology)

"The attachment for example, methyl groups (-CH3) to the DNA of a gene, usually to cytosine, prevents transcription to mRNA, by stopping the RNA polymerase from binding. The modification of histones by addition, for example of, methyl or acetyl groups, affects how tightly the DNA is wrapped around the histone. When wound tightly, the genes are inactive: they cannot be transcribed to mRNA. The gene therefore cannot make protein; it is 'switched off'."

Can you tell me what part "In terms of transcription" that I'm wrong please. I'm really stuck :frown:



Acetylation of histones does not cause compaction. It causes relaxation. And therefore more transcription.

Methylation is a completely different process. In fact it is DNA bases that get methylated, and his causes compaction. Which causes transcriptional silencing.
Original post by alkyone
Acetylation of histones does not cause compaction. It causes relaxation. And therefore more transcription.

Methylation is a completely different process. In fact it is DNA bases that get methylated, and his causes compaction. Which causes transcriptional silencing.


Yeah the book is a little misleading which is why I tried to gather notes from other resources.

So am I correct with what I said originally?...

"Modification of histones for example, by removing acetyl groups, makes DNA wrap tighter around the histone proteins (chromatin becomes highly condensed). This prevents transcription to mRNA as RNA polymerase can’t bind to DNA. So the gene is NOT expressed."
Original post by Philip-flop
Yeah the book is a little misleading which is why I tried to gather notes from other resources.

So am I correct with what I said originally?...

"Modification of histones for example, by removing acetyl groups, makes DNA wrap tighter around the histone proteins (chromatin becomes highly condensed). This prevents transcription to mRNA as RNA polymerase can’t bind to DNA. So the gene is NOT expressed."


Removal of acetyl groups means that lysine's positive side groups are exposed meaning that DNA can bind tightly so yes that's correct.
Original post by alkyone
Removal of acetyl groups means that lysine's positive side groups are exposed meaning that DNA can bind tightly so yes that's correct.


Thank you!! Your explanations are very clear! :smile:
Original post by Philip-flop
Thank you!! Your explanations are very clear! :smile:


That's ok :smile:
Original post by alkyone
Acetylation of histones does not cause compaction. It causes relaxation. And therefore more transcription.

Methylation is a completely different process. In fact it is DNA bases that get methylated, and his causes compaction. Which causes transcriptional silencing.


Actually either DNA bases or histones can be methylated. You're right that either causes silencing.
Original post by anosmianAcrimony
Actually either DNA bases or histones can be methylated. You're right that either causes silencing.


Just explaining what his book says :smile:

Edit: also it is best not to go into histone methylation because the outcome is not always transcriptional silencing. Depending on the position and combination of the aminoacids methylated, as well as number of -CH3 added you can have different outcomes.
(edited 7 years ago)
Reply 16
Avatar for Muz_333
Muz_333
1
As has been explained. Chromatin structure plays a huge role in gene expression. The tighter the chromatin structure the more the gene is repressed/silenced. However a gene within a loose state is not necessarily expressed, or on. In order for a gene to be induced above basal levels a transcriptional activator must be involved.

P.S. I'm drunk so take this with some salt, it's been a while since I've looked at histone structure. Lately I've been interested in gene looping to explain some gene expression differences for one of my papers I'm about to submit.
(edited 7 years ago)
Original post by anosmianAcrimony
Actually either DNA bases or histones can be methylated. You're right that either causes silencing.


Histone H3, lysine 4 trimethylation (H3K4me3) is associated with increased gene activity, and stuff like H3K27me3 and H3K9me3 are associated with repression.

It's important to realise all these processes are interlinked, for example, MeCP2 is a protein that binds to methylated CpG islands and recruits histone deacetylase complexes.

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